This invention relates to an apparatus and a system for controlling the drive of an electric motor by detecting the motor current.
In controlling the rotational speed of a poly-phase alternating current motor with a poly-phase rectangular voltage, the detection of the angular position of the rotor is required.
Conventionally, a method has been disclosed which controls a synchronous motor without directly detecting the electric angular position of the rotor in the electric motor by means of a sensor, but which estimates the positions of the magnetic poles of the synchronous motor. For example, JP-A-7-245981 discloses a method wherein a voltage pulse is applied to a permanent magnet synchronous motor (hereafter referred to as PM motor) so that the positions of the magnetic poles are estimated on the basis of a current pulse developed along the axis perpendicular to the axis along which the voltage pulse was applied. This method is applicable to starting a PM motor which is first at rest. JP-2001-251889 discloses a method of estimating on the basis of the current flowing through a PM motor the phase of a voltage induced due to the rotation of the PM motor.